A variety of implantable medical devices (IMDs) for delivering a therapy, monitoring a physiological condition of a patient or a combination thereof have been clinically implanted or proposed for clinical implantation in patients. Some IMDs may employ one or more elongated electrical leads carrying stimulation electrodes, sense electrodes, and/or other sensors. Other IMDs may incorporate electrodes and/or other sensors along or within a housing of the IMD that encloses circuitry and electronic components of the IMD.
IMDs may deliver therapy to and/or monitor conditions of a variety of organs, nerves, muscle or tissue, such as the heart, brain, stomach, spinal cord, pelvic floor, or the like. Some IMDs, such as cardiac pacemakers monitor a patient's heart activity and provide therapeutic electrical stimulation to the heart of the patient via electrodes coupled to the pacemaker. The electrical stimulation provided by the IMD may include signals such as pacing pulses to address abnormal cardiac rhythms such as bradycardia, tachycardia and fibrillation.
An IMD may sense signals representative of intrinsic depolarizations of the heart and analyze the sensed signals to identify normal or abnormal rhythms. Upon detection of an abnormal rhythm, the device may deliver an appropriate electrical stimulation signal or signals to restore or maintain a more normal rhythm. For example, an IMD may deliver bradycardia pacing, anti-tachycardia pacing (ATP), or cardioversion or defibrillation shocks to the heart upon detecting an abnormal rhythm. A pacing pulse will capture the heart, i.e., successfully cause a pacing-evoked depolarization of the myocardium, if the pacing pulse energy is equal to or greater than the capture threshold.
In some cases, the IMD senses a signal representative of the metabolic demand of the patient in order to provide cardiac pacing at a rate intended to meet the metabolic demand of the patient. For example, an indication of the patient's physical activity level may be determined from an accelerometer signal in order provide rate responsive pacing to dynamically maintain a heart rate that meets the metabolic demand of the patient.